1. Field of the Invention
This invention relates to an apparatus for welding multiple webs of synthetic thermoplastic material as they move around a deflecting roller, which apparatus comprises an elongate housing, which is movable toward the deflecting roller to assume a position in which said housing is approximately concentric to said roller and closely surrounds said roller around a part of its periphery, and said housing is provided with at least one longitudinally extending hot-air duct, which is curved in the same sense as the housing and is provided with an inlet for receiving flowing hot air and is formed with a plurality of outlet bores or gaps in that wall of the housing which faces the deflecting roller.
2. Description of the Prior Art
A known apparatus of that kind is shown in FIG. 1 of the drawing and comprises an approximately semicircular housing 3, which embraces a deflecting or reversing roller 1 on approximately one-half of its periphery. A multiple film seb or a continuous tubular film 2 of synthetic thermoplastic material is trained around said roller. The housing has milled openings, which constitute hot-air passages 4 and 5 and are separated by a wall at their inner ends, where they are provided with radial tubular ports 6. The tubular ports 6 are connected to the housing 3 by retaining elements 7. The tubular ports 6 constitute parts of a heater 8, which serves to heat air that has been blown into the heater through a tubular port 9. The heated air is forced into the passages 4 and 5 through the tubular ports 6. The heated air exits from the housing 3 through a multiplicity of small bores 10 and causes the two plies of the tubular film 2 to be welded to each other in the area with which the tubular film moves over the row of bores 10. A plurality of welding apparatuses constituted by respective housings 3 may be arranged one beside the other. A disadvantage of the known apparatus described with reference to FIG. 1 resides in that the heater 8 must have a high power to ensure that the air exiting from the bores 10 will be at a sufficiently high temperature for a satisfactory welding of the two film plies to each other. The fact that a high power is required for the heater 8 is due not only to the loss of radiant heat but also to the design of the heater. The latter consists of a solid block of iron, which is heated by a heating cartridges, which have been inserted into bores. Cold air from the tubular port 9 is blown through additional bores, which are provided in the iron block. That air is heated in said block and through the tubular ports 6 enters the housing 3 of the welding apparatus.